docs: def get_multi_causal_downstream(graph, nbunch: Union[BaseEntity, Iterable[BaseEntity]]):
      """Get the union of all of the 2-level deep causal downstream subgraphs from the nbunch.
  
      :param pybel.BELGraph graph: A BEL graph
      :param nbunch: A BEL node or list of BEL nodes
      :return: A subgraph of the original BEL graph
      :rtype: pybel.BELGraph
      """
      result = get_downstream_causal_subgraph(graph, nbunch)
      expand_downstream_causal(graph, result)
      return result